1. Field of the Invention
This invention relates to a process for the formation of a neutralized sulfonated polymer having about 10 to about 50 meq. sulfonate groups, wherein the sulfonate groups are neutralized with a zinc counterion. A cement of a polymer having olefinic functional groups and a non-reactive solvent is contacted with a sulfonating agent such as an acyl sulfate. The sulfonated polymer is then neutralized with a solution of zinc acetate dissolved in a solvent system of methanol alone or methanol and water, wherein the volume percent of methanol based on non-reactive solvent is less than about 10. A carboxylic acid can be then added to the cement of the neutralized sulfonated polymer thereby forming a zinc salt of the carboxylic acid in situ, wherein the zinc salt of the carboxylic acid is a preferential plasticizer for the zinc sulfonate groups. The Brookfield viscosity at 0.6 rpm at room temperature of the cement of the zinc neutralized sulfonated polymer is less than about 50,000 cps.
2. Description of the Prior Art
Recently, a new class of thermoelastic sulfonated polymers has been described in a number of U.S. patents. These sulfonated polymers are derived from polymeric materials having olefinic unsaturation, especially elastomeric polymers such as Butyl and EPDM rubbers. U.S. Pat. No. 3,642,728, herein incorporated by reference, clearly teaches a method of selective sulfonation of olefinic unsaturation sites of an elastomeric polymer to form an acid form of a sulfonated elastomeric polymer. The olefinic sites of the elastomeric polymer are sulfonated by means of a complex of a sulfur trioxide donor and a Lewis base. The SO.sub.3 H groups of the sulfonated elastomer can be readily neutralized on a two-roll mill with a basic material to form an ionically crosslinked elastomer at room temperature having substantially improved physical properties over an unsulfonated elastomer. These sulfonated elastomers, may be processed somewhat like a conventional thermoplastic at elevated temperatures under a shear force in the presence of selected preferential plasticizers which dissipate the ionic associations at the elevated temperatures thereby creating a reprocessable elastomer.
The basic materials used as neutralizing agents are selected from organic amines or basic materials selected from Groups, I, II, III, IV, V, VIB, VIIB and VIII and mixtures thereof of the Periodic Table of Elements.
Although this patent teaches the use of NaOH dissolved in methanol as a neutralizing agent, it fails to teach the criticality of the volume % of the methanol, the use of the zinc acetate to provide improved metal neutralized sulfonated polymers, or the ability to form a preferential plasticizer in situ.
U.S. Pat. No. 3,836,511, herein incorporated by reference, teaches an improved process for the sulfonation of the olefinic sites of the elastomeric polymer, wherein the improved sulfonating agent is selected from acetyl sulfate, propionyl sulfate and butyryl sulfate. The neutralizing agents employed to neutralize the acid form of the sulfonated elastomeric polymers in solution are organic amines dissolved in an aliphatic alcohol. The resultant neutralized sulfonated elastomeric polymers prepared by this process exhibit somewhat inferior physical properties due to a rather low degree of ionic association.
U.S. Pat. Nos. 3,870,841 and 3,847,854, herein incorporated by reference, teach a method of plasticization of the polymeric backbone of a neutralized sulfonated polymer. The plasticizing agent is incorporated into the sulfonated polymer by hot melting the neutralized sulfonated polymer with the plasticizing agent. Although the rheological properties are improved, the incorporation of these plasticizing agents into the neutralized sulfonated polymers is extremely difficult and usually results in a general decrease in physical properties.
The four aforementioned patents teach the method of compounding the additives into the neutralized sulfonated elastomeric polymer under high heat and shear conditions or the use of an organic amine neutralizing agent thereby resulting in compositions either having poor rheological or physical properties.
The present invention teaches a new improved process for the formation of an improved gel-free neutralized sulfonated elastomeric polymer having both excellent rheological and physical properties. The improvement in the process comprises the neutralization of the sulfonated cement by the addition of zinc acetate dissolved in a solvent system of methanol or a mixture of methanol and water which can be followed by the subsequent addition of a carboxylic acid to form in situ a zinc salt of the carboxylic acid which acts as a preferential plasticizer for the zinc neutralized sulfonate groups.